United States Nuclear Regulatory Commission - Protecting People and the Environment

IE Circular No. 78-11, Recirculation M-G Set Overspeed Stops


CR78011 

                               UNITED STATES 
                       NUCLEAR REGULATORY COMMISSION 
                         WASHINGTON, D. C. 20555 

                               June 7, 1978 

MEMORANDUM FOR:     B. H. Grier, Director, Region I 
                    J. P. O'Reilly, Director, Region II 
                    J. G. Keppler, Director, Region III 
                    K. V. Seyfrit, Director, Region IV 
                    R. H. Engelken, Director, Region V 

FROM:               Norman C. Moseley, Director, Division of Reactor 
                      Operations Inspection, Office of Inspection and 
                      Enforcement 

SUBJECT:            IE CIRCULAR 78-11, RECIRCULATION M-G SET OVERSPEED
					STOPS

     The subject circular is transmitted for issuance no later than the 
close of business on June 15, 1978. The circular should be issued to all 
holders of BWR Operating Licenses or Construction Permits. Also enclosed is 
a draft copy of the transmittal letter. 


                                   Norman C. Moseley, Director 
                                   Division of Reactor Operations 
                                     Inspection 
                                   Office of Inspection and Enforcement 

Enclosures: 
1.  Draft Transmittal Letter 
2.  IE Circular No. 78-11 

CONTACT:  C. J. DeBevec, IE 
          49-28180 
.

(Transmittal letter for IE Circular 78-11, to each holder of a BWR Operating
License or Construction Permit.) 

Addressee: 

The enclosed IE Circular No. 78-11, is forwarded to you for information. No 
written response is required. Should you have any questions related to your 
understanding of this matter, please contact this office. 

                                   Sincerely,  


                                   Signature 
                                   (Regional Director) 

Enclosures: 
1.  IE Circular No. 78-11 
2.  List of IE Circulars 
      Issued for 1978  
.

                             UNITED STATES 
                      NUCLEAR REGULATORY COMMISSION 
                   OFFICE OF INSPECTION AND ENFORCEMENT 
                         WASHINGTON, D. C. 20555 

                              June 15, 1978 

                                                      IE Circular No. 78-11 

RECIRCULATION M-G SET OVERSPEED STOPS 

In August 1977, personnel at the Cooper Nuclear Station reported finding the
recirculation M-G set overspeed mechanical and electrical stops at 112 
percent and 120 percent of rated core flow. The licensee had believed the 
set point to be 107 percent for these stops, although a search of Station 
preoperational and startup test records could not establish the actual 
initial set point. Subsequent to this finding, the licensee set the 
mechanical stop at 102 percent of rated core flow. 

The technical specifications require that the Minimum Critical Power Ratio 
(MCPR) be maintained greater than 1.22 for 8 X 8 fuel. The technical 
specifications further require MCPR to be above a minimum of 1.22 times Kf 
whenever operating at less than rated flow. The correction factor Kf is 
always greater than or equal to 1.0 and thus increases the MCPR limit when 
at less than rated flow. Technically, this factor increases the required 
MCPR thermal margin in the event a recirculation M-G set speed control 
failure would occur and allow the recirculation pump to overspeed. The lower 
the initial core flow the more severe is the flow transient, thus Kf 
increases as actual core flow is decreased. The Kf factor is read from a 
curve included in the technical specifications. 

A family of Kf curves is given in the figure in the technical 
specifications. The proper curve to use is the curve corresponding to the 
set point of the manual flow control overspeed stop (the M-G set mechanical 
stop). A lower set point of the overspeed stop results in a small Kf 
correction factor for a given core flow, as the flow transient on an M-G set 
speed control failure is less severe. Thus, it can be seen that by using a 
lower curve (e.g., 107, percent curve) than that required by the actual 
overspeed stop set point (believed to have been set at 112 percent), a 
nonconservative Kf correction factor was being used. 

The M-G set overspeed stops were set during the preoperational test program 
by a General Electric Company startup engineer. The stops were checked by a 
GE preoperational test procedure. This procedure did not include a 
requirement to specifically record the stop setting. It 

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IE Circular No. 78-11                                   June 15, 1978      

included only a requirement to verify that the stop setting had been made. 
It appears likely that these same procedures, and perhaps the same engineer,
were involved in the preoperational/startup test programs at several other 
sites. 

All holders of operating licenses for Boiling Water Reactor plants employing
recirculation M-G sets should be aware of the potential for improperly or 
undocumented settings on recirculation M-G set speed stops. It is 
recommended that the following be considered in your review: 

1.   Facility records should be searched to determine if the actual existing
     set point on the recirculation M-G set overspeed mechanical stops is 
     positively established. 

2.   If the setting of the mechanical and electrical overspeed stops is in 
     doubt, then steps should be initiated to determine the setting or 
     additional conservatism should be included in the determination of Kf. 
     3.   The actual Kf being used for the process computer calculations in 
     the determination of MCPR limits should be verified to be appropriate 
     to (or more conservative than) the actual setting of the recirculation 
     M-G set mechanical overspeed steps. 
     
This circular is also issued to holders of BWR Construction Permits for 
information or use as appropriate to assure that steps are taken during the 
preoperational test program to confirm that the M-G overspeed stops are set 
properly and that the proper Kf will be used with the process computer. 

No written response to this circular is required. If you require additional 
information regarding this matter, contact the Director of the appropriate 
NRC Regional Office. 

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